Automatic bypass valves are specifically, but not exclusively, applied in the field of apparatus controlling the outflow of high-pressure fluids such as, for example, fluids used in washing and cleaning plant machinery.
The invention finds particular application in the field of unloader valves which, when the flow to a user is stopped or reduced, apart from automatically bypassing the fluid, also maintain the fluid delivery branch to the user at a relatively low pressure. This means that the pumping system does not meet, when delivery is recommenced, any resistance due to the presence of a great localized pressure difference in the valve and downstream of it in the flexible delivery pipe, which has its own tap installed in a washing gun at the delivery end.
The prior art teaches valves of this type, essentially consisting in a three-way valve (pump inlet, delivery to user, pump tank return), internally of which a return obturator is provided, normally closed when delivery is under way and open when delivery has been interrupted.
The closing of the obturator is done automatically through a pressure difference between the two end surfaces of a piston connected to the obturator stem. This pressure difference exists only when there is a fluid downflow through the valve delivery outlet. This is made possible by the special conformation of the valve delivery chamber, internally of which a Venturi effect creates a zone of depression connected through an inclined conduit to one of the chambers housing the piston. The obturator is opened automatically when the delivery downflow stops, since the above-mentioned pressure difference is reduced.
Such valves present some drawbacks.
Firstly, a part of the energy possessed by the fluid is dissipated for the sole aim of generating the depression zone automatically controlling the obturator displacement.
Secondly, the forces coming into play regarding the automatic displacement of the obturator are of modest entity and are variable according to the fluid flowrate. This leads to extremely precise and accurate workmethods having to be used in manufacture of the valves; added to which, the nozzle producing the Venturi effect must be suited to the plant flow rate, leading to a need to have the same number of nozzles as utilizable flow rates. Furthermore, the behavior of the return obturator is highly sensitive to even minimum variations in the seal conditions of the piston. A slight increase in friction between the piston and the cylinder, due for example to a calcium deposit, or an infiltration of grit, can cause the mechanism to seize. Wear on the coupling, or hot water flow can generate excessive lateral leakage, reducing the total useful thrust on the piston.
A further drawback is represented by the long response times of the obturator in closing, when the delivery is reopened. This drawback can be obviated by creating a counterpressure in the return chamber, for example by means of an adjustable diaphragm or another choke mechanism, which however constitutes a constructional complication. In certain cases the use of a spring is necessary in order to aid the closing movement of the obturator.